High performance solar cells in general require low area contacts to minimise the contribution of the high recombination velocity metal/silicon interface to the device dark saturation current. Most laboratory solar cells use photolithographic techniques or laser scribing to facilitate the formation of low area metal contacts to the silicon that typically cover less than 1% of the solar cell surface. This minimises the device dark saturation current which in turn maximises the cell open circuit voltage (Voc). The highest Voc values ever achieved for silicon solar cells is in the vicinity of 720 mV, through careful attention to minimising the recombination taking place at the front and rear surfaces and particularly at the metal/silicon interfaces.
In comparison, the dominant commercial solar cell, namely the screen-printed solar cell shown schematically in FIG. 1, has a relatively large metal/silicon interface area in the vicinity of 10% for the front surface and often approaching 100% for the rear metal contact. Such a cell is shown schematically in FIG. 1 and comprises a silicon substrate 11, the bulk of which is lightly doped p-type, a thin top layer 12 lightly doped n-type to form a junction with the p-type region 11, front contact fingers 14 under which there is generally a heavily doped n-type region 13 and on the rear surface a continuous rear contact 15 makes connection with the p-type region 11. Such cells achieve relatively low efficiencies and voltages compared to high efficiency laboratory cells. To improve the performance of such cells, attention needs to be paid to improving the front and rear surface passivation and in particular, developing approaches for reducing the metal/silicon interface area for both metal contacts to much lower values.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.